This invention relates to internal reflectance spectroscopy using an immersion probe to enter a container whose contents are to be analyzed. The analysis is accomplished by attenuated total reflectance (ATR), for which an internal reflectance element (IRE) is used.
Two patents relating to this field of development have been issued to assignees of the present inventor. U.S. Pat. No. 4,835,389 disclosed an IRE at the lower end of a single cylindrical hollow tube. U.S. Pat. No. 5,051,551, assigned to the assignee of this application, disclosed an immersion probe comprising a pair of hollow light pipes which conduct IR radiation to and from an ATR element which may or may not also serve as the retroreflecting element.
The two light pipe design has a distinct performance advantage over the single light pipe design because it obviates the need to use a beamsplitter to separate the incoming and outgoing beams. For any realistic beam divergence, the beams are essentially scrambled by the light pipes. For a given optical aperture, beam splitting results in a loss of at least 50% for both the incoming and outgoing beams, giving a theoretical maximum transmission of 25%.
In spite of the transmission advantage of the two light pipe design, there are important potential applications which require probe diameters too small to be practical with two light pipes. For this reason, the applicant has developed a new probe employing a single light pipe. In doing so, it has been necessary to deal with a problem that was not addressed in U.S. Pat. No. 4,835,389.